ImgSaver: refactor to allow additional img types
ImgSaver: refactor to allow additional img types

This removes the tight integration of ImgSaver with PlanarImg.

The class has been generalized and now relies on two utility methods -
getting a plane count, and extracting an array for a given plane index.
These methods can now be extended to support additional Img types.

For future plans regarding Img compatibility, see:
https://github.com/scifio/scifio/issues/224

scifio/scifio